Fabrics and yarns that make up the back of the carpet as opposed to the carpet pile or face. In tufted carpet:

Primary backing - A woven or nonwoven fabric in which the yarn is inserted by the tufting needles.

Secondary backing - Fabric laminated to the back of the carpet to reinforce and increase dimensional stability.

In woven carpet, the backing is the “construction yarns” which are interwoven with the face yarn.